The mechanism of VLS (vapor-liquid-solid) growth of inorganic fibers, or filamentary "whisker" crystals of silicon and other materials, including silicon carbide, was first proposed by Wagner and Ellis in The Vapor-Liquid-Solid Mechanism of Crystal Growth and its Application to Silicon, Transactions of the Metallurgical Society of AIME, 233, 1053 (1965). According to the VLS mechanism, in the vapor deposition of crystalline material such as silicon onto a substrate, a droplet of liquid is situated between the vapor and the growing crystal. The surface of the liquid is a preferred site for deposition from the vapor. The liquid becomes supersaturated with material supplied by the vapor, and crystal growth occurs by precipitation at the solid-liquid interface.
The mechanism was supported by the observations that whiskers of crystals of Al.sub.2 O.sub.3, SiC, and BeO had rounded terminations at their tips, regrown silicon whiskers exhibited a liquid-like globule at the tip, and certain impurities were essential to silicon whiskers, rather than film, growth.
It was determined that the impurities identified were whisker growth promoting agents which formed a liquid solution with the crystalline material to be grown at the deposition temperature. This growth promoting agent was found in the hemispherical terminations of the tips of the VLS grown whiskers. Silicon was suggested as a growth promoting agent for SiC whiskers.
The formation of inorganic fibers, such as SiC, by the VLS process has been investigated by the contacting, with a reactive gas, of a growth promoter containing substrate at elevated temperature. The reactive gas contained components of the fiber to be deposited, such as silicon and carbon for SiC fibers. Generally, growth of the fibers has been observed roughly normal to the surface of the substrate, the growth promoter tip being disposed on the end of the fiber opposite the substrate.
For example, U.S. Pat. No. 3,519,472 describes the growth of silicon carbide whiskers having a knob of an alloy of iron and silicon, sometimes in combination with nickel, titanium or other metals, generated by the reaction of silicon monoxide, carbon monoxide and hydrogen over substrates of silicon nitride, silicon carbide, alumina or carbon, where the metl was present in the substrate or the refractory brick lining the high temperature reaction furnace.
Fiber growth promoter materials proposed include metals, alloys and compounds of iron, nickel, manganese, chromium and titanium.
Silicon carbide fibers or whiskers have shown suitability for use as reinforcing materials for composites, in which the matrix material may be polymeric resins, glasses, silicon, aluminum, copper or another ceramic, such as Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3.
In order to be commercially viable, SiC fibers produced by the VLS process must exhibit reproducibility regarding length and diameter, parameters which in previous embodiments have varied by as much as 100 percent from one production run to the next. U.S. Pat. No. 3,519,472 supra reports a tenfold difference in length to thickness ratios of from 10:1 to 100:1.
We have traced this problem to the preparation of the substrate material for the reaction, particularly the treatment of the substrate with fiber growth promoter material.
Various methods for treating the substrate with the fiber growth promoter material have been proposed. Application of the material onto a roughened substrate was proposed in U.S. Pat. No. 3,692,478; sprinkle, powder or vapor deposition in U.S. Pat. No. 3,721,732; and brush, dust or vapor deposition in U.S. Pat. No. 4,013,503. In U.S. Pat. No. 3,622,272 it was proposed to apply the material by brushing on the substrate a powdered metal suspended in a liquid vehicle, exemplified as an equal volume of acryloid resin and an alcohol. In U.S. Pat. No. 3,778,296, it was proposed that a layer of carbon (such as charcoal), growth promoter and synthetic or natural lacquer (in a solvent) be applied with a brush onto the substrate.
One conventional method for preparing substrates for SiC fiber production is dipping in or brushing onto a substrate, a binder mixture, such as polymethylmethacrylate and fumed silica in methylethylketone, and sprinkling onto the coated substrate, the fiber growth promoter as a metal powder.
These conventional methods of preparing substrates generally produce inhomogeneous substrates in which the particle size range of fiber growth promoter material is large, and the distribution of such particles over the substrate surface is disordered, often with gaps existing on the surface.
In addition, when the ratio of binder to promoter is too large, a less active carbon rich promoter is formed, and black undergrowth is formed on the substrate among the green beta-SiC fibers. When the ratio of binder to promoter is too small, the promoter material powders are not bound to the substrate, and are lost.
Furthermore, vapor deposition techniques require expensive capital instruments to carry out the deposition, with commercial scaleup being questionable.
It is necessary, therefore, for economic commercialization of fibers of the desired uniform qualities and characteristics that an inexpensive and a more reproducible method of preparing substrates be employed.